1. Field of the Invention
The invention relates to a turbomachine in which two concentric shafts are subject to mutual braking in the case of a failure.
2. Description of the Related Art
A specific example of this situation call be explained with reference to FIG. 1 which illustrates a well known aircraft engine. The engine comprises a rotor 1 surrounded by a stator 2 separated by a main tunnel 3 with an annular cross-section. The main tunnel 3 is occupied by blade stages that are alternately fixed to the rotor and the stator to accelerate and compress gases before taking advantage of the energy released by them as they expand after combustion of the fuel. Working from the front towards the back, there are seen blades of a low pressure compressor 4, blades of a high pressure compressor 5, a combustion chamber 6, blades of a high pressure turbine 7 and blades of a low pressure turbine 8. The rotor 1 is actually composed of two parts; a high pressure rotor body 9 supports the mobile blades of the high pressure compressor 5 and the high pressure turbine 7, and a low pressure rotor body 10 supports the mobile blades of the low pressure compressor 4 and the low pressure turbine 8. Furthermore, the high and low pressure rotor bodies 9 and 10 comprise respectively a high pressure shaft 11 and a low pressure shaft 12 that support them by means of bearings connected to stator 2; thus, working from the front towards the back, there is a front bearing 13 for the low pressure shaft 12, a front bearing 14 for the high pressure shaft 11, a rear bearing 15 for the high pressure shaft 11 and a rear bearing 16 for the low pressure shaft 12. These bearings include one or two ball or roller bearings as the active element, so that shafts 11 and 12 can rotate at high speed independently of each other. Concentric shaft 11 and 12 are completely separated, but it will be seen that they are only separated by a small clearance over a fairly long proximity area 17 located approximately adjacent to the front bearing 14 of the high pressure shaft 11.
Many modern engines have a high compression ratio and a large dilution ratio of combustion gases. They are provided with an auxiliary tunnel 18 surrounding the main tunnel 3, and the air passing through the auxiliary tunnel then mixes with the combustion gases at the back of the low pressure turbine 8. The air which passes along this auxiliary tunnel 18 is accelerated by the blades of a fan 19 fixed to the low pressure rotor body 10, which extends in front of the low pressure compressor 4. The diameter of the blades of the fan 19 is very large, and consequently their inertia is high. They are also the most exposed to bread age when a foreign body such as a bird accidentally enters into the engine.
A large out-of-balance mass appears on the low pressure rotor body 10 as soon as one of the fan blades 19 is broken, which induces very high vibrational forces on it, which are transmitted to the low pressure shaft 12 and the stator 2 through the front bearing 13. Damage caused by these excessive forces can propagate throughout the engine. This is why the front bearing 13 of the low pressure shaft 12 sometimes acts as a fuse if this type of failure occurs. In other words it would break or give way in some other manner.
Many types of frangible bearings are known in prior art, for example as disclosed in U.S. Pat. Nos. 5,417,501 and 5,433,584. They usually provide an incipient failure of stator 2 close to the front bearing 13 which separates the front bearing 13 from the rest of the stator 2; the part providing the incipient failure is usually a thin portion of the stator structure 2 or small diameter connecting bolts in which the threaded rod may be notched. The incipient failure part is designed to tear or break when an out-of-balance mass occurs, such that the front bearing 13 separates from stator 2 so that it no longer supports the low pressure shaft 12 which is then free to oscillate by tipping around the rear bearing 16 without producing any excessive forces on stator 2. In the meantime, the controller is informed of the failure and stops the engine, such that shafts 11 and 12 can gradually slow down and stop. Hopefully, the subsequent repair to the engine will be limited to replacement of the damaged fan blades and the frangible bearing.
Additional and much more serious damage can occur due to tipping of the low pressure shaft 12 around the rear bearing 16 if shafts 11 and 12 come into contact with each other near the proximity area 17 as shown in FIG. 2, since considerable overheating can then occur due to the friction resulting from the rotation speeds of these two shafts 11 and 12, which are very different and very high (for example 4500 rpm and 17000 rpm). The production of heat concentrated at a contact area 21 over a very small angular distance around the circumference of the shafts would be such that the slowest shaft, namely the low pressure shaft 12, could be damaged at this location, and its metallurgical state could be modified, reducing its strength, or it may even be broken or friction welded to the high pressure shaft 11. There is then a risk that the low pressure shaft could break such that the fan would be lost, or at least it would be necessary to replace the low pressure shaft 12 during the repair. Similarly, the high pressure shaft 11 could also be damaged, although it is apparently less vulnerable due to its higher rotation speed so that it passes in front of the contact area 21 which helps to distribute the increase in temperature around its circumference.